Glow cathode for cathode ray tubes



t f8, Rfic-sRLlcl-l Er AL VUIIDW "CTHODE FOR IGAT'ODE 'RAY TUBES sinduan. 17, 193s lmllll Patented Cet. 8, 1940 UNIT f D ST T GLO'W CATHODEFOR CATHODE RAY TUBES Application January 17, 1938, Serial No. 185,455ln Germany December 22, 1936 4 Claims.

The invention relates to improvements in glow cathodes for cathode raytubes, in particular Braun tubes, and is directed especially to glowcathodes provided with a high emission substance.

Cathodes of the type referred to have the disadvantage that the emissionsubstance evaporates gradually during the operation of the cathode raytube with the result that the emission capacity of the cathodedecreases.

It is the principal object of the invention to provide a high emissioncathode in which the evaporation of the emission substance is preventedas much as possible. This is accomplished by arranging the high emissionsubstance in the interior of a heated cavity and arranging in front ofthe cavity one or more electrodes positively biased with respect to thepotential applied to the recess. 'Ihe purpose of the electrodes is todrive back to the cathode any vaporised particles which leave thecathode in the form of positively charged ions. This action may beamplified by arranging additional positive electrodes in front of theglow cathode.

It is also an object of the invention to provide means for heating theelectrodes for retaining in this manner also the neutral particles inthe interior of the cavity or for regenerating the cathode.

Still another object of the invention is to cover also the heatableelectrodes in front of the cathode with an emission substance forproducing in this manner a very capacious glow cathode.

The emission substance employed comprises preferably a compound ofalkali metals, for instance a caesium compound, which is embedded in aweakly reducing or oxidising substance, as for instance the earthalkali-oxides. As examples of a caesium compound may be mentionedcaesium sulphate or caesium azide which is applied in the form of analcoholic paste which is permitted to dry in the air. When embedding thecaesium compound in a reducing or oxidising substance, the latter isconverted into a paste, for instance by dissolving it in alcohol,whereupon this paste is mixed with the caesium paste.

It is also possible to apply the alkali metals to the inner wall of thecavity and then apply on top of the same the oxide of the earth alkalimetal. The presence of the oxide of the earth alkali metal causes theproduction of alkali oxide which evaporates to a smaller degree than themetal.

It may also be more suitable to apply the emission substance only to thebottom of the cavity and to apply to the side wall of the cavity alining of a metal oxide which due to its higher emission power absorbsany alkali atoms striking the same. If aluminum oxide is employed forthis purpose, this material may at the same time be used as a heaterelement in which the electric heating wires are embedded.

In certain cases it may be advisable to provide the electrodes in frontof the cathode with a coating of a metal oxide for absorbing thevaporised alkali metal.

The accompanying drawing illustrates diagrammatically and by way ofexample a few embodiments of the present invention:

Fig. l shows an electron gun comprising a high emission glow cathode inlongitudinal section and a series of electrodes in front of the same,and

Fig. 2 shows a longitudinal section of a difierent construction of aglowcathode.

Referring to Fig. l, the body l of the high emission cathode is providedat one end with a cylindrical cavity 2 whose bottom and side wall islined with a layer of emission substance 3. The other end of the body lhas embedded therein a resistance wire fl forming the heating element ofthe cathode and supplied with electric current by the source 5.

In front of the cathode and in axial alinement with the cavity 2 of thesame is arranged in spaced relation from one another a series of screenelectrodes S, 1, 8 and 9 and a centrally apertured disc electrode l0.The electrodes E, l, 8, 9 and l are connected with positive voltage tapsof a potentiometer il connected across the terminals of a source l2 andthus are positively biased with respect to the cathode.

In the embodiment of the invention illustrated in Fig. 2, the body i@ ofthe glow cathode is provided with a tubular cavity 2a. The electrodes M,I5 and I6 arranged in this cavity constitute the high emission elementsof the cathode. The electrode lll consists of a circular disc providedat its circumference with a cylindrical flange, and the electrodes l5and i6 consist each of a centrally apertured disc with a cylindricaliange at their outer circumference. The inner faces of the disc of theelectrodes lll, l5 and it have applied thereto a layer 29 of emittingsubstance while the inner wall faces of the flanges of these electrodesare lined with a metal oxide 2i.

In front of the outermost emitting electrode iE and axially spacedtherefrom is arranged an apertured disc anode i9 having at its outercircumference a cylindrical ange.

A potentiometer ila connected across the termnals of a source ofpotential l2a is provided with a plurality of taps connected with theelectrodes M, l5, i and I9.

Between each two electrodes is arranged an annular member i3 consistingof insulating material, as for instance a ceramic substance. The tubularbody le of the cathode is surrounded by a heating coil indicated at 22and supplied with current from the source 5a.

If desired the individual electrodes may be formed and used as electriclenses and/or may be treated to cause the emission oi secondaryelectrons for increasing the efficiency of the electrodes.

What we claim is:

i. In a cathode ray device, an electron gun comprising a high emissionglow cathode formed ofl a cathode body having at one end a cavity, anemission substance applied to the Walls of said cavity, means forheating .said cathode body, a centrally apertured disc electrode axiallyspaced from said cathode body and at least one screen electrode mountedbetween said cathode and disc electrode in front of said cavity, andmeans positively biasing said screen electrode with respect to saidcathode body so as to return any positive ions which tend to leave saidcavity.

2. In a cathode ray device, an electron gun comprising a high emissionglow cathode formed of a cathode body having at one end a cavity, anemission substance applied to the walls: of said cavity, means forheating said cathode body, a centrally apertured anode in axialalinement with said cathode body, a plurality of axially alined andaxially spaced screen electrodes mounted 'in iront of said cavity andbetween said cathode body and said anode, and means positively biasingeach said screen electrodes with respect to potential applied to saidcathode body and with respect to the adjacent screen electrode which iscloser to said cavity.

3. In a cathode ray device, an electron gun comprising a high emissionglow cathode formed of a cathode body having at one end a cavity, anemission substance applied to the walls of said cavity, means forheating said cathode body, a centrally apertured anode, a plurality ofaxially alined and axially spaced screen electrodes mounted in iront ofsaid cavity and between said cathode body and said anode, said screenelectrodes being provided with a coating of emission substance, andmeans positively biasing each of said' screen electrodes with respect topotential applied to said cathode body, said last named means applying asuccessively higher potential to said screen electrodes in proportion totheir distance from said cathode body.

4. In a cathode ray device, an electron gun comprising a high emissionglow cathode formed of a cathode body having at one end a cavity, anemission substance applied to the walls of said cavity, means forheating said cathode body, a centrally apertured anode, a plurality ofaxially alined and axially spaced screen electrodes mounted in front of`said cavity and between said cathode body and said anode, said screenelectrodes having oxidized surfaces` for absorbing vaporized particlesof said emission substance, and means positively biasing each saidscreen electrodes with respect to the potential applied to said cathodebody, said last named means applying a successively higher potential tosaid screen electrodes in proportion to their distance from said cathodebody.

PAUL GRLICH. MARTIN PLOKE.

